<!DOCTYPE html>
<html>
<head lang="en">
    <meta charset="UTF-8">
    <title>练习</title>
</head>
<body>
    <script>
      var m=1;
      for(m=1;m<7;m++){
          if(m!=3)
          document.write("<h"+m+">"+m+"级标题"+"</h"+m+">");
          else{
              document.write("<h"+3+">"+"三级标题"+"</h"+m+">");
          }
      }
    var a1=10;
    var a2=20;
    alert("a1+a2="+a1+a2);
    </script>
</body>
</html>